The invention relates to nematic liquid-crystal compositions of negative dielectric anisotropy (xcex94∈) based on substituted 2,3-difluorophenylene, in particular based on substituted 4-(trans-1,4-cyclo-hexylene)-2,3-difluorophenylene, and substituted trans-4-(trans-1,4-cyclohexylene) cyclohexylenecarboxylic esters. These compositions are particularly suitable for plasma-addressed display devices.
Plasma-addressed displays (PADs) are of great utility for economically interesting high-information displays. Such PADs are used, for example, in TV applications and, for example in displays for computer screens, automobiles and aircraft.
PADs have electrical switching elements and a plasma cell with a plurality of addressing channels which is connected to the display cell. Such PADs are disclosed, for example, in WO 96/00925, EP 0 628 944, EP 0 545 569, U.S. Pat. No. 4,896,149 and U.S. Pat. No. 5,077,553.
In a PAD, the switching elements are addressed by means of a time multiplexing scheme. This charges the electrodes of a pixel in the limited time during which they are active. They subsequently become and remain inactive until they are addressed again in the next cycle. Consequently, the change in voltage at a plasma-addressed pixel in this inactive time is a not undesired, but very crucial characteristic of such a display. Discharge of the electrodes of the pixel is determined by two factors, namely the capacitance of the pixel (C) and the specific resistance of the liquid-crystal material between the electrodes (R), i.e. of the liquid crystal. The time constant for the experimental drop in voltage is proportional to RC and is known as the RC time.
PADs make particular demands of the dielectric properties of the liquid-crystal compositions. Both xcex94∈, as in conventional TFT-AMDs, and ∈∥ and ∈xe2x8axa5 are of crucial importance here. This applies both to PADs in TN mode and in particular to those in ASM mode. Since PADs can be produced in considerably larger screen diagonals compared with conventional active matrix displays (AMDs) with, for example, thin-film transistors (TFTs) as active switching elements, they are suitable, as the first liquid-crystal displays ever, for achieving the long-held target of a wall-hung TV set (Buzak, Information Display 4 and 5 (1998, pp. 22 ff.). TV sets with PADS having a 25 inch diagonal are commercially available and prototypes of displays with a 42 inch screen diagonal have already been produced (Burgmans, Kakizaki and Uede, Information Display 4 and 5 (1998), pp. 14 ff.). With these large screen diagonals, the problem of the relatively high dependence of the contrast and the hue on the viewing angle, often abbreviated to the viewing angle dependence, of the conventionally utilized effect of the twisted nematic (TN) cell becomes predominant. At least with these screen diagonals, but generally already for screen diagonals in the range from 17 to 20 inches, the viewing-angle dependence of TN displays is prohibitive for market success. For this reason, work has been in progress for some time on various solutions for improving the viewing-angle dependence. For PADs, the use of the so-called axially symmetrically aligned microcell (ASM), which allows a viewing-angle range of 140xc2x0 both horizontally and vertically, is particularly suitable (Burgmans et al.). In particular for ASM-PADs, dielectrically negative liquid-crystal media are suitable. It is now been found that liquid-crystal materials of negative dielectric anisotropy are suitable for ASM-PADs.
It has been found that liquid crystals of negative xcex94∈ are particularly suitable for PADs in ASM mode.
There is thus a great demand for liquid-crystal compositions having high specific resistance and other material properties suitable for use in PADs, in particular in. ASM-PADs, such as, for example, a broad nematic mesophase range with an extremely low smectic-nematic transition temperature and no crystallization at low-temperature, and negative dielectric anisotropy.
A further requirement in such compositions is for a low steepness of the characteristic lines (large difference between V90 and V10) for addressing grey shades, where a greater addressing voltage, for example compared with TN-AMDs, can be accepted.
An object of the invention is to provide liquid-crystal compositions of very high specific resistance which also satisfy the other requirements.
Upon further study of the specification and appended claims, further objects and advantages of this invention will become apparent to those skilled in the art.
An additional requirement is for compositions of low An values of xe2x89xa60.1, preferably xe2x89xa60.09, particularly preferably xe2x89xa60.08, and rotational viscosities of xe2x89xa6150 mpaxc2x7s at 20xc2x0 C., preferably xe2x89xa6120 mPaxc2x7s, in particular xe2x89xa6100 mPaxc2x7s.
It has now been found that nematic liquid-crystal compositions for plasma-addressed liquid-crystal displays (PALCDs) comprising at least one compound of the formula I 
in which
R11 and R12, independently of one another, are alkyl or alkoxy having 1 to 8, preferably 1 to 5, carbon atoms or alkenyl, alkenyloxy or alkoxyalkyl having 2 to 8, preferably 2 to 5, carbon atoms,
Z11 and Z12, independently of one another, and if more than one Z11 are present, these too independently of one another, are a single bond, xe2x80x94(CH2)2xe2x80x94, transxe2x80x94CHxe2x95x90CHxe2x80x94 or xe2x80x94CH2Oxe2x80x94,
A11 and A12, independently of one another, and if more than one A11 are present, these too independently of one another, are
trans-1,4-cyclohexylene, 1,4-cyclohexenylene, in which, in addition, 1 or 2 non-adjacent xe2x80x94CH2xe2x80x94groups may be replaced by oxygen atoms, 1,4-phenylene, 2-fluoro-1,4-phenylene, 3-fluoro-1,4-phenylene, 2,3 difluoro-1,4-phenylene, 3,5-difluoro-1,4-phenylene, 2,6-difluoro-1,4-phenylene or 1,5-pyridyl or 1,5-pyrimidyl, in which one or two H atoms may be replaced by F,
where preferably at least one of the rings A11 and A12 is a trans-1,4-cyclohexylene ring and particularly preferably two of the rings A11 and A12 are trans-1,4-cyclohexylene, and
n is 0, 1 or 2, preferably 0 or 1,
are particularly suitable for PAD applications.
In PADs, very high values for the RC time, i.e. the time constant for discharge of the pixel electrodes, can be achieved. The high values for the RC time, or the voltage holding ratio, which corresponds to this quantity, are achieved on one hand by a thin installation sheet, but on the other hand are also determined essentially by the type and composition of the liquid-crystal material used. These compositions likewise have reduced viscosity and exhibit no crystallization and no smectic phases at temperatures down to xe2x88x9220xc2x0 C., preferably at temperatures down to xe2x88x9230xc2x0 C., particularly preferably at temperatures down to xe2x88x9240xc2x0 C., when stored in test TN-cells with a layer thickness corresponding to an optical retardation of about 0.5 xcexcm format least 100 hours. They can preferably be stored for at least 500 hours, very particularly preferably for at least 1000 hours.
Preference is furthermore given to liquid-crystal mixtures comprising at least one compound of the formula II 
in which
R21 and R22, independently of one another, are alkyl or alkyloxy having 1 to 8, preferably 1 to 5, carbon atoms or alkenyl, alkenyloxy or alkoxyalkyl having 2 to 8, preferably 2 to 5, carbon atoms, and
A21 is trans-1,4-cyclohexylene, 1,4-phenylene, 3-fluoro-1,4-phenylene, 2-fluoro-1,4-phenylene or 2,3-difluoro-1,4-phenylene.
Particular preference is given to nematic liquid-crystal compositions which comprise at least one compound of the formula III 
in which
R31 and R32, independently of one another, are alkyl or alkyloxy having 1 to 8, preferably 1 to 5, carbon atoms or alkenyl, alkenyloxy or alkoxyalkyl having 2 to 8, particularly preferably 2 to 5, carbon atoms, 
xe2x80x83are each, independently of one another,
and, if more than one 
xe2x80x83is present, these too independently of one another, are trans-1,4-cyclohexylene, 1,4-phenylene, 3-fluoro-1,4-phenylene or 2-fluoro-1,4-phenylene, and
Z31, Z32 and Z33, each independently of one another, and, if more than one Z31 are present, these too independently of one another, are CH2CH2, COO, OCO, CH2O, OCH2 or a single bond, and
m is 0, 1 or 2, with the proviso that m is not 1 at the same time as 
Z31 is a single bond and Z32 is COO.
Particularly preferably, m=0 or 1, Z32 and, if present, Z31 are a single bond 
and 
and, if present, 
are trans-1,4-cyclohexylene or 1,4-phenylene.
The liquid-crystal composition very particularly preferably comprises one or more compounds selected from the group of compounds of the formulae III1 to III2
in which the parameters are as defined above for the formula III and are preferably Z32 and Z33, independently of one another, are xe2x80x94CH2CH2xe2x80x94 or a single bond, 
are each, independently of one another, trans-1,4-cyclohexylene or 1,4-phenylene, and one of these rings is alternatively 2-fluoro-1,4-phenylene or 3-fluoro-1,4-phenylene,
and/or a compound of the formula III3
in which the parameters are as defined above for the formula III and are preferably one of
Z31 and Z32 is xe2x80x94CH2CH2xe2x80x94, xe2x80x94COOxe2x80x94 or a single bond and the other is a single bond, and 
are trans-1,4-cyclohexylene or 1,4-phenylene.
The mixture particularly preferably comprises one or more compounds selected from the group consisting of the compounds III1a to III1d: 
in which, in each case independently of one another,
n and m are an integer from 1 to 7, preferably from 1 to 5,
l is 0 or 1, preferably 0, and
p and q are each independently an integer from 0 to 3, preferably 0 or 1.
Preference is furthermore given to mixtures comprising compounds selected from the group consisting of the compounds of the formulae III2a to IIIc and/or III2b to III2g and/or III2h to III2i: 
in which n, m, p and 1 are each as defined above for the formulae III1a to III1d.
Preference is also given to mixtures comprising one or more compounds from the group consisting of the compounds of the formulae III3a to III3d: 
in which the parameters are as defined for the formula III.
The mixtures particularly preferably comprise:
a) one or more compounds of the formula III1a, in which l is preferably 1, and/or
b) one or more compounds of the formula III1a in which l is 0 and one or more compounds of the formula III1a in which l=1;
c) one or more compounds of the formula III1c and/or III1d, but preferably of the formula III1c, in which l is preferably 0;
d) one or more compounds of the formula III2h,
e) one or more compounds of the formula III3a in which R31 and R32 are preferably alkyl having 1 to 5 carbon atoms;
f) one or more compounds of the formula IIa and/or IId, described below;
g) one or more compounds of the formula IIa, described below;
h) one or more compounds of the formula IId, described below;
Preference is furthermore given to nematic liquid-crystal compositions which, besides two or more compounds of the formula I, comprise at least one compound of the formula II: 
in which the parameters are as defined above for the respective formulae.
In a preferred embodiment, the compositions according to the invention comprise a total of from 30% to 70%, preferably from 40% to 65%, of two or more, preferably from three to seven, compounds of the formula I.
The nematic compositions particularly preferably comprise at least two compounds of the formula I selected from the group consisting of the formulae Ia to Ib: 
in which, each independently of one another, n and m are an integer from 1 to 7, preferably from 1 to 5, and l is 0 or 1, preferably 0.
The nematic compositions preferably comprise at least one compound selected from the group consisting of the compounds of the formulae IIa to IId: 
in which R31 and R32 are as defined above for the formula II, but are preferably n-alkyl having 1 to 5 carbon atoms.
The LC compositions according to the invention have a dielectric anisotropy, measured at 20xc2x0 C. and 1 kHz, of from xe2x88x922.0 to xe2x88x925.0, preferably between xe2x88x922.5 and xe2x88x924.3, in particular from xe2x88x922.9 to xe2x88x923.9. ∈∥ is preferably from 2.5 to 4, particularly preferably from 3.0 to 3.8, very particularly preferably from 3.1 to 3.7. The liquid-crystal compositions according to the invention have a birefringence of not more than 0.1, preferably not more than 0.093, particularly preferably not more than 0.08, very particularly preferably not more than 0.076, at 20xc2x0 C. and 589 nm. The voltage holding ratio (VHR), measured after 5 minutes at 100xc2x0 C. and at a measurement voltage of 1 V using an Autronic Melchers measuring instrument in TN test cells with a layer thickness of about 5 xcexcm with AL-1041 (Nissan Chemicals, Japan) as alignment layer, is, in a preferred embodiment, at least 83%, preferably at least 90%, particularly preferably at least 95%. In this embodiment, the RC time in the finished PAD is significantly increased, in a preferred manner, by the thin sheet. In another preferred embodiment, in which the improvement in the RC time in the PAD is optional, the voltage holding ratio in said TN test cells is at least 98%, preferably greater than 98.5%, particularly preferably greater than 99.0%, very particularly preferably at least 99.5%. The clearing point of the compositions according to invention is above 75xc2x0 C., in particular above 80xc2x0 C., very particularly above 90xc2x0 C.
Compositions of this type preferably comprise two, three or four compounds of the formula I. Preferred compositions comprise at least 10% by weight or more of each of the individual compounds of the formula I.
The compounds of the formulae I, II and III are known to the person skilled in the art. See, e.g., U.S. Pat. No. 4,229,315 and U.S. Pat. No. 5,087,764.
Preferred compositions likewise comprise two or more compounds of the formulae III. Preference is given here to the compounds from the group consisting of the formulae III selected from the formulae IIIa and IIIb: 
where R31 and R32 are as defined above for the formula III and are preferably, each independently of one another, alkyl having 3 to 5 carbon atoms.
In a preferred embodiment, the compositions according to the invention comprise at least two compounds of the formula IIIb.
The compositions according to a further preferred embodiment of the present invention comprise from 6 to 48% by weight, in particular from 10 to 34% by weight or from 16 to 30% by weight, of components of the formula I, based on the mixture as a whole. The proportions of the other groups in per cent by weight is preferably in the following ranges:
formula II: from 0 to 30%, in particular from 3 to 30%
formula III: from 10 to 60%, in particular from 20 to 50%.
The components of the formulae I, II and III preferably form the basis of the claimed compositions and together make up at least 80% by weight, particularly preferably at least 90% by weight, of the compositions. However, it is likewise possible to use other LC components in smaller percentages in addition to the components of the formulae I to III in order to fine-tune the claimed compositions. Particular use is made here of compounds of high clearing point and/or high birefringence.
The liquid-crystal compositions according to the invention can, if necessary, also comprise further additives, such as, for example, dichroic dyes and in particular chiral dopants, in conventional amounts. The concentrations of these further additives are 0-10% in total, preferably from 0.1 to 5% in total, in particular from 0.1 to 3% per compound employed, in each case based on the mixture as a whole. The concentration of these additives is not taken into account when indicating the composition of the liquid-crystal mixture and in the rules for setting the dimensioning of their concentration ranges.
The nematic liquid crystal compositions of the present invention preferably comprise
a) two or more compounds of the formula I in which n=0, R11 is alkyl and R12 is alkoxy,
b) two or more compounds of the formula I in which n=1, R11 is alkyl and R12 is alkoxy,
c) one or more compounds of the formula I in which n=0, R11 is alkyl and R12 is alkoxy, and one or more compounds of the formula I in which n=1, R11 is alkyl and R12 is alkyl or alkoxy,
d) one or more compounds of the formula I in which n=0, R11 is alkyl and R12 is alkoxy, and one or more compounds of the formula I in which n=1, R11 is alkyl and R12 is alkoxy,
e) one or more compounds of the formula I in which n=0, R11 is alkyl and R12 is alkoxy, one or more compounds of the formula I in which n=1 and R11 and R12 are alkyl, and one or more compounds of the formula I in which n=1, R11 is alkyl and R12 is alkoxy,
f) one or more compounds of the formula IIa and/or IId,
g) one or more compounds of the formula IIa,
h) one or more compounds of the formula IId,
i) one or more compounds of the formula III1a,
j) one or more compounds of the formula IIIa,
k) one or more compounds of the formula IIIb,
l) one or more compounds of the formula III1a and one or more compounds of the formula IIIa,
m) one or more compounds of the formula III1a , one or more compounds of the formula IIIa and one or more compounds of the formula IIIb,
n) one or more compounds of the formula I, one or more compounds selected from the group consisting of the formulae IIa and IId and one or more compounds selected from the group consisting of the compounds of the formulae III1a, III1c, III1d, III2a, IIIa and IIIb, or
o) one or more compounds of the formula III1c, particularly preferably with l=0, n=1 to 5, and p=0, 1, 2 or 3, of these preferably p=0.
In general, the compositions consist of a plurality of compounds mixed in a conventional manner. In general, the desired amount of the components used in a lesser amount are dissolved in the components making up the principal constituent, advantageously at elevated temperature. If the selected temperature is above the clearing point of the principal constituent, completion of the dissolution process is particularly easy to observe.
However, it is also possible to prepare mixtures in other known ways, for example by using so-called pre-mixes, for example homologue mixtures, or multibottle systems.
By means of suitable additives, the liquid-crystal phases according to invention can be modified in such a way that they can be used in any type of ASM-PAD that has been disclosed hitherto.
Unless stated otherwise, all temperatures above and below, thus including, in the examples, the melting point, the smectic-nematic phase transition temperature and the clearing point of a liquid-crystal substance, are given in xc2x0 C., the temperature differences are given in differential degrees Celsius, the percentages are per cent by weight, and the physical properties are the values at 20xc2x0 C.
In order to characterize the liquid-crystalline compounds used in the compositions according to invention, the following abbreviations are used in the examples. The indices n and m denote the number of carbon atoms present in the terminal alkyl radicals.
In the present application and in the examples below, the structures of the liquid-crystal compounds are indicated by means of acronyms, the transformation into chemical formulae taking place in accordance with Tables A and B below. All radicals CnH2n+1 and CmH2m+1 are straight-chain alkyl radicals having n and m carbon atoms respectively. The coding in Table B is self-evident. In Table A, only the acronym for the parent structure is given. In individual cases, the acronym for the parent structure is followed, separated by a dash, by a code for the substituents R1, R2, L1, L2 and L3.
The physical properties of the liquid-crystal mixtures were determined as described in xe2x80x9cPhysical Properties of Liquid Crystalsxe2x80x9d, Ed. M. Becker, Merck KGaA, as of Nov. 1997, unless explicitly stated otherwise.
The following examples are intended to illustrate the invention without representing a limitation. Above and below, percentages are per cent by weight. All temperatures are all given in degrees Celsius. xcex94n denotes the optical anisotropy (589 nm, 20xc2x0 C.), xcex94∈ the dielectric anisotropy (1 kHz, 20xc2x0 C.), H.R. the voltage holding ratio (at 100xc2x0 C., after 5 minutes in an oven at 1 V), V0 the capacitive threshold voltage determined at 20xc2x0 C. and 1 kHz. The low-temperature stability of the nematic liquid-crystal compositions was investigated by storage tests in sealed TN cells with a layer thickness of about 5 xcexcm. The alignment layer used was CU-1511 from Merck DuPont, USA. The test cells were bonded to polarizers for easier observations of the phase behavior and stored in storage cabinets at certain temperatures (xe2x88x9220xc2x0 C., xe2x88x9230xc2x0 C. or xe2x88x9240xc2x0 C.). The cells were assessed visibly for phase conversion or structural changes at intervals of about 24 hours. The figure given for the storage stability tstore (T) was the time of the final observation just before any change was evident.
The entire disclosure of all applications, patents and publications, cited above, and of corresponding Germany patent application No. 19841832.9, filed Sep. 12, 1998 is hereby incorporated by reference.